The present invention relates to a method of controlling the orientation of a smectic liquid crystal, particularly a chiral smectic liquid crystal, used in preparation of a liquid crystal device such as a liquid crystal display device and a liquid crystal optical shutter array and, more particularly, to a method of controlling the orientation of a liquid crystal for improving display and driving characteristics by the initial orientation or alignment of liquid crystal molecules, a device used in the method and a liquid crystal device prepared by the method.
Hitherto, liquid crystal display devices have been well known, which comprise a group of scanning electrodes and a group of signal electrodes arranged in matrix, and a liquid crystal compound filled between the two electrode groups to form a plurality of picture elements or pixels to display images or information at or near matrix intersecting points. For driving these display devices, there is employed a time-sharing driving method comprising selectively applying address signals sequentially and periodically to the group of scanning electrodes and selectively applying certain information signals to the group of signal electrodes in a parallel fashion in synchronism with the address signals. However, these display devices and the driving method therefor have serious drawbacks which will be described below.
Namely, it is difficult to obtain a high density of picture elements or large image area. Because of their relatively high response speed and low power dissipation, most liquid crystals which have been put into practice as display devices are TN (twisted nematic) type liquid crystals, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, "Applied Physics Letters" Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128. In the liquid crystal of this type, molecules of nematic liquid crystal which show positive dielectric anisotropy under no application of an electric field form a structure twisted in the thickness direction of the liquid crystal layers (helical structure), and molecules of this liquid crystal form a structure aligned or oriented parallel to each other near the surfaces of both electrodes. On the other hand, the nematic liquid crystal which shows positive dielectric anisotropy under application of an electric field is oriented or aligned in the direction of the electric field, thus enabling optical modulation. When display devides are constructed in a matrix electrode arrangement using a liquid crystal of this type, a voltage higher than a threshold level required for aligning liquid crystal molecules in the direction perpendicular to electrode surfaces is applied to an area (a selected point) where a scanning electrode and a signal electrode are selected at a time, whereas a voltage is not applied to areas (non-selected points) where scanning electrodes and signal electrodes are not selected. Accordingly, the liquid crystal molecules are stably aligned parallel to the electrode surfaces. When linear polarizers having a cross-nicol relationship to each other (i.e. their polarizing axes are arranged perpendicular to each other) are arranged on upper and lower sides of the liquid crystal cell thus formed, light is not transmitted at selected points while it is transmitted at non-selected points. Thus, the liquid crystal cell can function as an image device.
However, when a matrix electrode arrangement is formed, a certain electric field is applied to regions where a scanning electrode is selected and signal electrodes are not selected or regions where a scanning electrode is not selected and a signal electrode is selected (which regions are so called "half-selected points"). If the difference between a voltage applied to the selected points and a voltage applied to half-selected points is sufficiently large, and a voltage threshold level required for allowing liquid crystal molecules to be aligned or oriented perpendicular to an electric field is set to a value therebetween, display devices normally operate. However, as the number (N) of scanning lines increases, a time (duty ratio) during which an effective electric field is applied to one selected point when a whole image area (corresponding to one frame) is scanned decreases with a ratio of 1/N. Accordingly, the larger the number of scanning lines, the smaller is the voltage difference as an effective value applied to a selected point and non-selected points when scanning is repeatedly effected. As a result, this leads to unavoidable drawbacks of lowering of image contrast or occurrence of interference or crosstalk. These phenomena are regarded as essentially unavoidable problems appearing when a liquid crystal having no bistability (i.e., liquid crystal molecules are horizontally oriented with respect to the electrode surface as the stable state and are vertically oriented with respect to the electrode surface only when an electric field is effectively applied) is driven (i.e., repeatedly scanned) by making use of a time storage effect. To overcome these drawbacks, the voltage averaging method, the two-frequency driving method, the multiple matrix method, etc. have been proposed. However, these methods are not sufficient to overcome the above-mentioned drawbacks. As a result, it is the present state that the development of large image area or high packaging density in respect to display elements has been delayed because it is difficult to sufficiently increase the number of scanning lines.
Meanwhile, turning to the field of a printer, as means for obtaining a hard copy in response to input electric signals, a Laser Beam Printer (LBP) providing electric image signals to electrophotographic photosensitive member in the form of light is most excellent in view of density of picture elements and printing speed.
However, the LBP has drawbacks as follows.
(1) The device becomes large in apparatus size.
(2) There is a high speed mechanically movable part such as a polygon scanner, resulting in noise and requiring strict mechanical precision, etc.
In order to eliminate drawbacks stated above, a liquid crystal shutter array as a device for changing electric signals to optical signals is proposed. When picture element signals are given with a liquid crystal shutter-array, for instance, more than 3000 signal generators are required for writing picture element signals into the length of 210 mm at a rate of 16 dots/mm. In order to independently feed signals to respective signal generators, wiring of lead lines for feeding electric signals to all of the respective signal generators is required, resulting in difficulties in production.
In view of the above, another attempt has been made to apply image signals corresponding to one line in a time-sharing manner with signal generators correspondingly divided into a plurality of rows. With this attempt, signal feeding electrodes can be common with a plurality of signal generator, thereby enabling a remarkable decrease in the amount of wiring required. However, if an attempt is made to increase the number (N) of rows using a liquid crystal showing no bistability as usually practiced, a signal "ON" time is substantially reduced to 1/N. This results in difficulties that the light quantity obtained on a photosensitive member is decreased, a crosstalk occurs, etc.
To overcome drawbacks with such conventional liquid crystal devices, the use of liquid crystal devices having bistability has been proposed by Clark and Lagerwall (e.g., Japanese Laid-Open Patent Appln. No. 56-107216, U.S. Pat. No. 4,367,924, etc.). In this instance, as the liquid crystals having bistability, ferroelectric liquid crystals having chiral smectic C-phase (SmC*) or H-phase (SmH*) are generally used. These liquid crystals have bistable states of first and second stable states with respect to an electric field applied thereto. Accordingly, in contrast to optical modulation devices in which the above-mentioned TN-type liquid crystals are used, the bistable liquid crystal molecules are oriented to first and second optically stable states with respect to one or the other electric field vectors, respectively. The characteristics of the liquid crystals of this type are such that they are oriented to either of two stable states at an extremely high speed and the states are maintained when an electric field is not supplied thereto. By making use of such properties, these liquid crystals having chiral smectic phase can essentially improve on a large number of problems with the prior art TN-type devices. This will be described in detail hereinafter in relation to the present invention.
However, in order that an optical modulation device in which a liquid crystal having bistability is used can realize desired driving characteristics, it is required that a liquid crystal disposed between a pair of parallel base plates has a molecule arrangement such that molecules can effectively be switched between the two stable states independent of the application of an electric field. For instance, in connection with ferroelectric liquid crystals having SmC*- or SmH*-phase, it is required that there is formed a region (monodomain) where liquid crystal layers having SmC*- or SmH*-phase are vertical to the surface of the base plates, i.e., the liquid crystal axis is aligned substantially in parallel therewith. However, with optical modulation devices in which a liquid crystal having bistability is used, the orientation of the liquid crystal having such a monodomain structure has not satisfactorily been formed, thus failing to obtain sufficient display characteristics.
For instance, in order to obtain such an orientation, there have been proposed a method of applying a magnetic field, a method of applying shearing stress, etc. However, these methods do not necessarily provide satisfactory results. For instance, the method of applying a magnetic field has drawbacks in that it requires a large scale apparatus and is not comparible with a thin layer cell having excellent operational characteristics. Further, the method of applying a shearing stress has a drawback in that it is not compatible with a method of filling a liquid crystal after a cell is prepared.
Meanwhile, in the liquid crystal device in which the above-mentioned TN-type liquid crystal is used, in order to form a monodomain of liquid crystal molecules in parallel with the surface of a base plate, for instance, a method of rubbing the surface of the base plate with a cloth, etc., or a method of effecting oblique vapor deposition of SiO, etc. has been used. In accordance with the rubbing method, liquid crystal molecules assume a low energy (i.e., stable) state where they align preferentially along the rubbing direction. Thus, a certain effect for preferentially orienting liquid crystals in one direction is given to such a rubbed surface. A structure having a face to which such wall effect is given is shown in, e.g. Canadian patent No. 1010136, etc. by W. Helfrich and M. Schdat. In addition to the rubbing method for giving the orientation effect, another method is employed, in which a structure having a face formed by oblique or tilt vapor deposition of SiO or SiO.sub.2 on a base plate is used, and the face having a uniaxial anisotropy of SiO or SiO.sub.2 has an effect for preferentially orienting liquid crystal molecules in one direction.
As stated above, the alignment or orientation control method, e.g., the rubbing method, or the oblique deposition method is one of the preferable methods for producing liquid crystal devices. However, if the orientation control is implemented to liquid crystals having bistability with these methods, a face having a wall effect for preferentially aligning a liquid crystal only in one direction is formed, thereby to deteriorating the desirable characteristics of the bistable liquid crystals such as bistability with respect to an electric field applied thereto, high responsiveness or monodomain forming ability.